<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <h1>1</h1>
    <button>+1</button>
    <script>
        var h1 = document.querySelector('h1')
        var btn = document.querySelector('button')
        btn.onclick = function(){
            h1.innerHTML = h1.innerHTML - 0 +1
        }
    </script>
</body>
</html>